Special safety conditions must be complied with when producing HA as thermal instability is one of its properties, i.e. it decomposes in an explosion, especially in the presence of metal ions, in strongly alkaline media, and at high temperatures and concentrations. The thermal composition of HA causes irreversible losses and reduces product yield.
The main ways of improving the method for producing HA are: producing highly pure HA, especially for the electronics industry; increasing efficiency of the HA production method and increasing the safety of the HA production method.
A method for producing a highly pure stabilized aqueous solution of free HA by treating an aqueous, anion-containing solution of free HA with an anion exchanger in alkaline form and mixed with a HA stabilizer is known from German patent DE 19936594 A1. The alkaline form of the anion exchanger is prepared using any base, particularly ammonia.
The disadvantage of this known method is its high cost due to the high price of the anion exchanger. In addition, the method is not highly efficient, in particular when its use is limited to solutions for the electronics industry.
U.S. Pat. No. 5,472,679 describes a method for producing an aqueous HA solution by reacting a solution of hydroxyl ammonium sulfate (HAS) with a suitable base at a temperature of ca. 60° C. The mixture obtained is then subjected to distillation under reduced pressure at a temperature below 65° C. There are considerable losses in HA due to its thermal decomposition at higher temperatures. A solid bottom residue is obtained (when releasing the HA formed salt), and the distillate is an aqueous solution containing 16 to 23% of HA.
This method does not provide sufficient technological safety. Water is separated during evaporation resulting in an increased HA concentration. It is known that the inclination of HA to decompose increases when its concentration rises to >70 percent by weight. There is a growing risk that the high HA concentration initiates the explosive decomposition of HA.
In addition, this method for producing an aqueous HA solution is not sufficiently reliable in plant operation because the solid bottom residue (e.g. sodium sulfate) accumulates on the inner walls of the apparatuses and forms deposits that are difficult to remove, impair the function of the apparatuses, and create the conditions for equipment failure. The setbacks of this method also include insufficient efficiency. Vacuum pumps or vapor exhausting devices are required for performing the distillation under reduced pressure, which increases both energy and operating costs of the method.
A method for producing an aqueous solution of free HA by treating a HA salt with a base, separation of the solution obtained by distillation into an aqueous HA fraction and a salt fraction at temperature above 80° C. and subsequent reconcentration of the aqueous HA solution by stripping it with vapor in a distillation column is known from patent publication WO 97/22551.
It is a disadvantage of this method that the HA solution is contaminated with sodium ions if caustic soda is used as a base. The presence of a metal ion in the product creates the risk of explosive decomposition of HA, which characterizes the low degree of technological safety this known method has. In addition, salt (sodium sulfate) can be precipitated at the bottom of the stripping column, which reduces the reliability of plant operation, requires thorough monitoring of the technological parameters, and considerably increases the costs of this method.
A disadvantage of the known method, if ammonia is used as a base, is insufficient HA yield of about 60-70%. This is because ammonia is a weaker base than caustic soda, for example, which completely pushes HA out of HA sulfate. In the known method, more than 30% of HA are discharged in the form of HAS as waste from the bottom of the distillation column and thus prevented from further use. The significant HA losses with wastewaters and the necessity of treating this waste considerably increase the expenses (costs) of the known method, making its industrial-scale application inefficient.
A method of processing HA-containing solutions by stripping the HA with steam is known from patent publication DE 10004818. Its disadvantage is its high energy demand as the method involves distillation in a vacuum. This known method lacks the use of intrinsic energy.
A method for producing an aqueous solution of free HA by simultaneous countercurrent treatment of an HA salt solution with ammonia or ammonia water as a base, separation of the solution obtained into an aqueous HA solution and a salt fraction by distillation, reconcentration of the aqueous HA solution by distillation with vapor in a distillation column is known from patent publication WO 99/07637. Analogous applications are DE 197 33 681 A1 and EP 1 012 114.
The major disadvantages of this method are insufficient technological safety and increased HA losses due to its thermal decomposition. Insufficient safety of the method is due to the physical properties of HA and process specifics of this method. Furthermore, safety conditions for the known method are not ensured over the entire range of pressure below (under a vacuum) and above atmospheric pressure. When using the known method under a vacuum, thermal decomposition of HA and its losses are insignificant but there is a risk that ambient air is sucked in when the reactive distillation column is relieved and an explosive air/ammonia mixture can develop in the reactive distillation column. In addition, reaction of HA with oxygen in the air produces nitrites that increase the risk of an explosion.
This risk of local formation of an explosive ammonia/air mixture when the column operated under vacuum is relieved is present not only in the reaction but also in the distillation part of the column. Using the known method, if there is excess ammonia (excluding HAS losses) in the reaction part of the column, it is stripped and discharged in the gaseous phase from the head of the column. The presence of ammonia both in the reaction and in the distillation parts of the column considerably reduces the technological safety of the known method when performed under a vacuum.
If the known method is performed at increased pressure, i.e. a pressure above atmospheric pressure, formation of an explosive ammonia/air mix is excluded (when the column is relieved), however, another risk arises: When the process temperature is raised above 100° C., HA losses increase significantly according to the conditions of the vapor/liquid equilibrium, and the risk of explosion arises from the thermal decomposition of the HA. If steam is used at a pressure of about 1.5 bara (at a temperature of ca. 130° C.) in the known method, local overheating of the HA solution occurs in the reaction part of the column, resulting in increased HA separation and a decrease in HA yield. This reduces the productivity of the HA production process until the vapor changes into thermodynamic equilibrium with the solution. Areas of local overheating of the HA solution may even occur at the considerable height of the reaction part of the column.
A decrease in safety of the known method for producing HA therefore results in the following disadvantages:
Increase of the thermal decomposition of HA with the risk of explosion due to the raised temperature of the process above atmospheric pressure.
Reduction of the HA yield due to increased thermal decomposition.
Risk of formation of an explosive ammonia/air mixture and of nitrites when relieving the column that is operated below atmospheric pressure (under a vacuum).
It is the problem of the invention to improve the safety of the method for producing aqueous solutions of free HA and to increase the yield of free HA.